In manufacturing integrated circuits, color filters, liquid crystal display elements, etc., finely patterning technology has been required and, in order to satisfy the requirement, there have been used radiation sensitive resist compositions capable of forming a pattern with high resolution. Formation of such fine pattern using the radiation sensitive resist composition is conducted, for example, in the following manner. A radiation sensitive resist composition is coated on a substrate such as a silicon substrate by a well-known or known coating method such as a spin coating method, roller coating method or the like, prebaked to form a radiation sensitive resist layer on the substrate, then patternwise exposed through a mask using radiation such as ultraviolet rays, deep ultraviolet rays, X-rays, electron beams or the like or directly beam-exposed without using a mask to thereby form a patternwise latent image in the resist layer, followed by post baking for improving developability, etching resistance, etc. and subsequent development processing. The thus formed resist pattern is subjected to optional post steps such as a dry etching step, a diffusing step, etc. As the resist to be used in this finely patterning technology, there have been known various positive- or negative-working ones.
Typical examples of conventionally known positive- or negative-working resists are illustrated below. As the positive-working resists, there are illustrated resists comprising an alkali-soluble resin and a quinonediazide photo-sensitizer and chemically amplified resists and, as the negative-working resists, there are illustrated polyvinyl cinnamate, an aromatic azide compound, a resist comprising a cyclized rubber and a bisazide compound, a diazo resin, a photopolymerizable composition containing an addition polymerizable unsaturated compound, a chemically amplified negative-working resist comprising an alkali soluble resin, a cross-linking agent and an acid generator, and the like. For example, in the above-described positive-working resist comprising an alkali soluble resin and a quinonediazide photo-sensitizer, a novolak resin not containing low molecular weight portion is preferably used as the alkali soluble resin from the resolution point of view. The resist containing such novolak resin, however, has the defect of insufficient sensitivity. Therefore, it is popularly conducted to add a sensitizer to a resist composition to improve sensitivity. This sensitizer is generally a low molecular weight phenol compound, and does not contribute to increasing heat resistance of the resist due to its low molecular weight. Therefore, when the resist is subjected to a high temperature processing step such as a post exposure baking step (PEB) or a dry etching step at a temperature of about 120 to 140.degree. C., deformation of the resist pattern takes place or the pattern edge portion becomes round to form a semicylindrical shape due to its low heat resistance. This phenomenon such as pattern deformation is observed seriously in a pattern portion where a large area is left as such. The reason for deformation of pattern in the pattern portion where a large area is left as such is generally explained as follows. That is, when a resist is prebaked, a solvent in the resist is removed by the prebaking heat. However, since drying of the resist proceeds from the resist surface, the interior of the resist is liable to contain a larger amount of the residual solvent. Then, the prebaked resist is made selectively soluble in a developer by irradiation with radiation. A resist pattern having a small area comparatively difficultly suffers from deformation of pattern dimension since it is supported by the surface of the resist pattern even when the interior of the resist shows some fluidity. However, with a resist pattern having a large area , the resist surface showing only a slight fluidity fails to support the pattern dimension when the interior of the resist shows some fluidity upon being heated, thus deformation of the pattern taking place. This problem of insufficient heat resistance arises not only with the novolak resin from which a low molecular portion has been fractionated but with novolak resins from which the low molecular portion has not been fractionated or other conventionally known resist materials. Deformation of a resist pattern or formation of a semicylindrical pattern in the high temperature processing would make it difficult to form a pattern with a desired width or resolution by etching, that is, control of dimension would become difficult. Thus, in order to render the resist pattern more heat resistant, so-called UV curing has been conducted in which a high energy radiation such as radiation with deep UV rays or radiation with UV rays in vacuo is conducted to thereby make the resist highly polymerized with the least loss of transmitted energy, thus imparting a high heat resistance. In the conventional art, however, the above problem has not still been fully solved.
Therefore, an object of the present invention is to provide a radiation sensitive resist composition not having the problem with conventional compositions and capable of forming a pattern with high sensitivity and high resolution and high heat resistance.